CryptMsgCalculateEncodedLength

[DllImport("Crypt32.dll", SetLastError = true)]
public static extern bool CryptMsgCalculateEncodedLength(
   uint dwMsgEncodingType,
   uint dwFlags,
   uint dwMsgType,
   IntPtr pvMsgEncodeInfo,
   IntPtr pszInnerContentObjID,
   uint cbData,
   ref uint pcbEncoded
);

Microsoft documentation: Link